This invention relates to automatic control valves used in the process industries such as refineries, chemical plants, power plants, and other industries that need to throttle gaseous or liquid media in response to electronic or pneumatic controlling signals in order to maintain a desired flow rate, tank level, process temperature, or other controlled variables.
All of these valves have to throttle and the controlled throttling area inside the valve has to vary at a mathematically pre-determined rate as function of the positioning signal to an actuating device and, therefore, the valve travel. Such a relationship is called "flow characteristic." It is typically accomplished by machining a contoured profile onto a movable valve plug which is penetrating a circular orifice or valve seat. The amount of penetration of the plug is a function of the actuating signal (and valve travel).
For certain reasons more fully discussed below, it can be more advantageous to machine such a throttling profile onto the outer extension of the seat ring bore. Such a modification of a basic control valve trim is exhibited in my prior invention, U.S. Pat. No. 3,304,949.
The purpose o the present invention is to provide a globe style control valve which offers the highest possible flow capacity with the smallest possible body opening. It should be realized that the weight of a valve, and therefore its manufacturing cost, varies nearly to the cube of its bonnet opening necessary to insert the valve plug and other parts of construction. A 2" valve having a conventional 2" valve plug and orifice will need an access opening of at least 2.5" to facilitate the internal working parts of a present state of the art control valve. My invention can provide an identical maximum flow capacity with an orifice diameter of only 1.5", therefore requiring an access opening of only 1.6", and thereby causing a weight reduction of nearly 74%. Such important weight savings are even more important when the valve material has to be a corrosion resistant alloy as is quite often the case in the chemical process industry.
A modulating control valve has basically two somewhat opposite requirements: First, it has to produce a high degree of turbulence in order to produce a sufficient conversion from kinetic to thermal energy (pressure reduction). Secondly, it has to have sufficient flow capacity when wide open to pass the mass flow that is generated at reasonable velocities by the connecting pipe size. To fulfill these requirements, a valve plug and seat ring should generate an abrupt flow pattern in the intermediate or low travel (throttling) positions to reduce pressure and a very streamlined and low fluid resistance producing profile at the maximum rated travel position in order to achieve a high flow capacity.